Fluid catalytic cracking units (FCC), have been widely utilized in refining crude oil derived feedstocks into gasoline and middle distillates. Their counterparts, i.e., heavy oil cracking units (HOC) also have been widely used for cracking of heavy reduced crudes, either with prior hydrotreating or without, to produce transportation fuels. In the FCC and HOC processes, crude oil derived feeds are contacted with a silica containing catalyst at high temperature in a catalytic cracking zone which generally is a riser or fluidized dense bed reactor and the catalyst is withdrawn continuously from the cracking zone and sent to a regenerator for removal of coke deposited on the catalyst. FCC processes differ slightly from HOC processes in that the FCC process generally results in less carbon being deposited on the catalyst and therefore less heat is generated. Also, the temperature rise is less in the regenerator. Catalysts used for FCC and HOC have little resistance to deactivation when excessive temperatures are reached in the regenerator.
In recent times, it has been proposed to convert FCC units to process heavy reduced crude oil feeds. Equipment modifications have been made to accommodate the removal of increased coke content deposited upon the catalyst during the cracking phase and to accommodate the removal of the increased heat generated during the regeneration phase so that the temperature limits for the FCC units and catalysts are not exceeded. FCC and HOC regenerators requiring simultaneous regeneration of the catalyst and removal of excess heat generated generally utilize one of two systems for the removal of the excess heat. One system involves the removal of catalyst from the regenerator to an external or side cooler with return of the cooled catalyst to the regenerator, and the second utilizes cooling systems within the regenerator itself. Illustrative patents showing these concepts include the following:
U.S. Pat. No. 4,615,992 discloses a process for hydrocarbon conversion in association with a catalyst regenerator, the catalyst regenerator having a dense phase fluid catalyst combustion bed zone and a catalyst disengagement zone. The unit contains a catalyst distributor for receiving and distribution spent catalyst within the upper portion of a fluidized catalytic bed. Air is introduced into the lower portion of the vessel to oxidize the carbon or coke deposited on the catalyst and effect catalyst regeneration. An externally mounted catalyst cooler is connected to the regenerator vessel with the catalyst cooler containing an air distributor at or near the bottom for maintaining catalyst fluidization within the cooler. Air used for fluidization of the catalyst effects modest regeneration in the cooler and is returned to the upper portion of the regenerator while cooled catalyst is removed from the bottom of the catalyst cooler and reintroduced at a point near the lower portion of the dense phase of the fluidized catalyst bed zone.
U.S. Pat. No. 4,064,039 discloses a fluid catalytic cracking unit system utilizing a platinum group metal modified cracking catalyst in conjunction with a regenerator and side mounted catalyst heat exchange cooler to permit adjustment of cracking conditions independent of the heat produced in the regeneration of the catalyst.
U.S. Pat. Nos. 4,434,245; 4,353,812; and 4,439,533 disclose hydrocarbon conversion processes wherein the catalyst is first removed from a regenerator and cooled in side or external heat exchange coolers and then returned to the regenerator. As noted in these patents, the method for controlling heat removal in the regenerator involves the extent of immersion of the cooling coils in the dense phase regenerated catalyst bed or rate of flow of regenerated catalyst through the external coolers.
U.S. Pat. No. 2,436,927 discloses a fluidized catalytic conversion process wherein the crude feed is contacted with a silica-alumina type catalyst for producing high quality gasoline. Crude oil feed is contacted in a reactor and the spent catalyst withdrawn from the reactor and charged to a regenerator for removal of carbon deposited upon the catalyst. Heat removal is achieved through the use of an external cooler and control is achieved by regulating the amount of catalyst passing through that cooler.
U.S. Pat. Nos. 3,990,992 and 4,219,442 illustrate regenerator units having heat removal means different from those described above. These regenerator units are divided into two portions; the regenerator having a lower portion for effecting combustion of the catalyst and an upper section wherein residual combustion is effected along with heat removal. Heat removal is achieved through internal coils in the upper section of the reg. Temperature control is achieved by controlling the amount of regenerated catalyst removed to the upper zone and then reintroduced along with coke contaminated catalyst to the combustion zone. The balance of the regenerated catalyst is reintroduced to the catalytic reactor.